1. Field of the Invention
The present invention relates to a toner used with electrophotography, and more particularly, to a toner used with electrophotography and having improved developing efficiency in which two types of silica particles with different particle sizes, hydrophobic titanium dioxide microparticles, and conductive titanium dioxide/positively chargeable aluminum oxide particles are used as external additives.
2. Description of the Related Art
As a developing method used in electrophotography, there are a two-component developing method and a one-component developing method. The one-component developing method is subdivided into a magnetic one-component developing method and a non-magnetic one-component developing method. The magnetic one-component developing method is a method that uses a magnetic one-component developing toner. The non-magnetic one-component developing method is a method that transfers a non-magnetic one-component developing toner layer formed on a developing roller to a photoconductor using a contact or non-contact developing method.
Among various developing methods using a one-component developer, a contact-type, non-magnetic, one-component developing method has excellent cost competitiveness, but provides poor dot reproducibility and line reproducibility, and low resolution, which renders creation of high quality images difficult. On the other hand, with respect to a non-contact-type, non-magnetic, one-component developing method, a simple developing apparatus is used, which promotes miniaturization of the apparatus. Also, this method enables color reproducibility, edge reproducibility, good tone gradation, and high resolution printing, thereby providing high quality images.
A non-contact-type, non-magnetic developing method includes a charging operation, a light-exposing operation, a developing operation, a transferring operation, and a cleaning operation. In the case of a toner used in a conventional non-contact-type, non-magnetic developing method, a colorant, a charge control agent, and a releasing agent are uniformly dispersed in a binder resin to improve color, chargeability, and fixing properties. Also, various types of external additives are added to toner particles to impart functionality, such as flowability, charge stability, and cleaning properties, to the toner particles.
Japanese Patent Laid-Open Publication No. Hei. 11-0095486 discloses a toner containing conductively treated silica particles as an external additive. Japanese Patent Laid-Open Publication No. Hei. 11-295921 discloses a toner containing three types of silica particles with different particle sizes as an external additive.
In the non-contact-type, non-magnetic, one-component developing method, it is important to form a thin toner layer on a developing roller opposite to a developing area, to prevent a fog (or background) in a non-image area and toner scattering. However, due to an increased charge amount of a toner during forming of a thin toner layer on a developing roller, developing efficiency is rapidly lowered, thereby decreasing an image density. Also, when the charge amount of a toner is adjusted to a low level to prevent the lowering of a developing efficiency, the increase of fog formation and contamination by toner scattering are likely to occur.
Therefore, stable maintenance of the charge amount and charge distribution of a toner as well as formation of a thin toner layer on a developing roller opposite to a developing area is required. This is accomplished by appropriately selecting the type and composition of external additives added to toner particles.